Background: Fragile X mental retardation syndrome (FXS) is the most common familial cause of intellectual disability and the most common known cause of autism. Other symptoms of FXS include depression, sensory processing deficits, aggressive behavior, connective tissue problems and digestive difficulties. This disorder arises when the number of CGG-repeats in the 5 UTR of the FMR1 gene exceeds 200. Such alleles become silenced. This results in a deficiency of the protein product of this gene, FMRP, which is involved in the regulation of translation of a subset of mRNAs. The FMRP deficiency in brain results in aberrant dendritic spine morphology and a defective response to synaptic activation. The mechanism of gene silencing is unknown. Progress report: We have previously identified a late event in the silencing process and shown that reversal of this event enables gene reactivation (Biacsi, Kumari and Usdin, 2008). In the past year we have been focusing on events that occur earlier in the silencing process. We have shown that in addition to histone H3 dimethylated at lysine 9 (H3K9Me2) and trimethylated at lysine 27 (H3K27Me3), silenced alleles are associated with elevated levels of histone H3 trimethylated at lysine 9 (H3K9Me3) and histone H4 trimethylated at lysine 20 (H4K20Me3). All four of these modified histones are present on exon 1 of silenced alleles at levels comparable to that seen on pericentric heterochromatin. H3K9Me2 and H3K27Me3 have a broad distribution across the 5'end of the gene, and may arise from spreading from an upstream heterochromatic zone we have identified 5'of the FMR1 gene. In contrast, H3K9Me3 and H4K20Me3 have a more focal distribution with the highest level of these marks being present in the vicinity of the repeat. This distribution provides clues as to the mechanism of gene silencing and suggests that the trigger for this silencing may be local to the repeat itself and may involve a mechanism similar to that involved in the formation of pericentric heterochromatin.